1. Field of the Invention
The present invention relates to a method for speed stage shifting of an automatic transmission in a vehicle such as an automobile.
2. Description of the Prior Art
The automatic transmission in a vehicle such as an automobile is automatically shifted among several speed stages according to changes of running conditions of the vehicle so as to provide a most desirable gear ratio available by a reduction gear mechanism incorporated therein at each instant of operation of the vehicle. The running conditions used to determined the speed stage shifting are basically vehicle speed and engine power output conventionally expressed in terms of throttle opening. As well known in the art, the speed stage is shifted from the lowest speed stage such as the first speed stage to a next to the lowest speed stage such as the second speed stage and then further to a next higher speed stage such as the third speed stage and so on as the relative magnitude of vehicle speed to throttle opening increases. In the modern automatic transmission of automobiles incorporating electronic computer control systems, the control of the speed stage shifting is more sophisticated than merely to shift up or down the speed stages according to the relation in magnitude between the vehicle speed and the throttle opening.
In order to accomplish the speed stage shifting with a high quality of smoothness with no minute abrupt change of torque in the rotational power transmission system extending from an engine to driving vehicle wheels by engagement or disengagement of friction engaging means such as clutches and brakes for changing over the route of transmittance of torque in a composite gear mechanism of the transmission, the timing control for the engagement or disengagement of the friction engaging means is essential. Those patterns which provide a high quality speed stage shifting differ according to whether the vehicle is running in an engine driving condition or a inertially running condition when the speed stage of the transmission is shifted.
In FIG. 1, an example is shown with respect to how the rotation speed of the engine or the input rotational member of the transmission changes in the process of an upshifting of the transmission such as from the second speed stage to the third speed stage, according to the disengagement of a first friction engaging means which has been engaged for providing the second speed stage and the engagement of a second friction engaging means which is engaged for providing the third speed stage, thereby generating a change in the output torque of the transmission. When the computing section of the automatic transmission decides the upshifting from the second speed stage to the third speed stage at time point t1, the disengagement of the first friction engaging means is immediately started so that the hydraulic pressure supplied to the first friction engaging means is exhausted through a passage system including an accumulator to present a time-based pressure performance pattern as shown by A in FIG. 1. In certain conventional computer controlled automatic transmission, the computer section judges at the same time or around as it decides the upshifting if the vehicle is in an engine driving condition or an inertially running condition. If the vehicle is in the engine driving condition, the supply of the hydraulic pressure to the second friction engaging means is immediately started at time point t1. Therefore, the hydraulic pressure in the second friction engaging means increases through a passage system incorporating an accumulator to present a time-based pressure performance pattern as shown by B in FIG. 1. According to the gradual progress of the engagement of the second friction engaging means the rotation of the engine is gradually decelerated as shown by a solid line in FIG. 1 under the application of a braking action due a partial engagement of both the first and second friction engaging means, and at time point t3 the rotational speed Ne of the engine is reduced to a rotation speed Ns which provides the same vehicle speed in the third speed stage as it provided in the second speed stage. The supply of the hydraulic pressure to the second friction engaging means is scheduled to place the second friction engaging means into its full engagement at time point t4 behind the time point t3.
When the computing section of the automatic transmission judges that the vehicle is running in an inertially running condition at the same time or around as it decides the upshifting from the second speed stage to the third speed stage, the supply of the hydraulic pressure to the second friction engaging means is delayed as shown by a performance pattern C in FIG. 1. In the inertially running condition, when the first friction engaging means is disengaged, the rotational speed of the engine starts to decrease by itself as shown by a broken line in FIG. 1 even with no application of the braking action due to the engagement of the second friction engaging means. Therefore, in this case it is desirable that the engagement of the second friction engaging means is delayed to avoid too quick deceleration of the engine which would generate an engine braking.
However, in fact, as shown in FIG. 2, in the relation between the vehicle speed and the throttle opening, there is generally an indefinite zone between the engine driving region and the inertially running region which is difficult for the computing section of the automatic transmission to infallibly discriminate the vehicle running condition between the engine driving condition and the inertially running condition. Therefore, if the computing section judged the vehicle is running in the inertially running condition and delays the supply of the hydraulic pressure to the second friction engaging means as shown by the performance pattern C when the vehicle is, however, in a slightly engine driving condition, the rotational speed of the engine will not decrease as expected by the broken line in FIG. 1 but will, for example, remain as shown by a dot-dash line in FIG. 1. In this case, when the second friction engaging means comes into its full engagement at time point t5, the engine is abruptly decelerated and a high peaky rise of torque is generated in the output shaft. On the other hand, if the second friction engaging means is engaged according to the performance pattern B when the vehicle is in a virtually inertially running condition, the engine is decelerated too much as shown by a two dots-dash line in FIG. 1, thereby generating an engine braking and an abysmal fall of torque at the instant of substantial engagement of the second friction engaging means.